1. Field of the Invention
The present invention relates to a HIT (Heterojunction with Intrinsic Thin layer) solar cell and a method of manufacturing the same and, more particularly, to an ultra-thin HIT solar cell and a method of manufacturing the same.
2. Description of the Related Art
Typically, a solar cell is a key element for photovoltaic power generation to directly convert solar light into electricity, and may be referred to a p-n junction diode.
In the course of converting solar light into electricity by a solar cell, solar light is incident on the p-n junction of the solar cell to produce electron-hole pairs, and electrons are moved to an n layer and holes are moved to a p layer by an electric field, so that photovoltaic power is generated in the p-n junction. As such, when a load or system is connected to both ends of the solar cell, an electric current may flow to thus produce electric power.
A solar cell is generally configured such that a p-type semiconductor layer is formed on an n-type silicon substrate. The p-type semiconductor layer is formed by doping of a p-type impurity. Thus, an n-type semiconductor layer is provided under the silicon substrate, and a p-type semiconductor layer is provided on the substrate, thus forming a p-n junction. The front and the back side of the silicon substrate are formed with metal electrodes to collect a small number of carriers (holes) and a large number of carriers (electrons) photo-produced by the p-n junction.
To develop such a solar cell, passivation properties of the surface of a silicon substrate are improved to thereby minimize the recombination rate of the small number of carriers, ultimately maximizing photovoltaic conversion efficiency of the solar cell.
Recently, HIT (Heterojunction with Intrinsic Thin layer) solar cells are being developed, wherein an amorphous silicon (a-Si) thin film having superior passivation properties and high electrical conductivity due to tunneling of carriers is disposed between a silicon substrate and an electrode, and an intrinsic amorphous silicon thin film is added between the silicon substrate and the amorphous silicon thin film, thus drastically increasing solar cell efficiency.
The HIT solar cell developed by Sanyo, Japan includes advantages of both amorphous silicon and monocrystalline silicon. The intrinsic amorphous silicon thin film is an amorphous silicon thin film layer close to the intrinsic state where the number of electrons and the number of holes are the same, thereby preventing recombination of electrons and holes due to interfacial defects between the crystalline silicon substrate and the amorphous silicon thin film.
In the HIT solar cell, a p-n junction may be formed by a high bandgap (about 1.7 eV) of a-Si (amorphous Si:H) containing hydrogen, resulting in high voltage. Recombination of charges may be decreased due to a high passivation effect by hydrogen passivation of a-Si:H, and both the front and the back side of the cell may absorb light, thus ensuring a high-efficiency solar cell. Furthermore, this solar cell is adapted for fabrication of an ultra-thin solar cell that is recently receiving attention.
Recently, a solar cell is intended to diffuse incident light through surface texturing to increase the usage efficiency of incident solar light. A HIT solar cell is increased in light trapping capacity through surface texturing.
FIG. 11 illustrates a typical HIT solar cell.
As in a typical crystalline silicon solar cell, a currently available HIT solar cell is configured such that both surfaces of an n-type crystalline silicon substrate 10 are wet etched or dry etched so as to be textured, intrinsic a-Si:H passivation layers 21, 22 are formed on the both surfaces, a p-type a-Si:H layer 31 and an n-type a-Si:H layer 32 are formed on the passivation layers 21, 22, transparent conductive oxide (TCO) layers 41, 42 comprising indium tin oxide (ITO) are formed thereon, and then an upper electrode 50 and a lower electrode 60 are formed thereon.
The HIT solar cell is advantageous because the technically advanced surface texturing technique of crystalline silicon may be applied, and the amorphous silicon layer such as the passivation layer is a very thin film and thus a total surface texture of the HIT solar cell may be formed through surface texturing of the crystalline silicon substrate.
However, etching both surfaces of an expensive crystalline silicon substrate may increase the solar cell manufacturing cost, and environmental problems attributed to wastewater and toxic gases generated in the wet and dry etching processes may occur. The surface defects of the textured crystalline silicon substrate may cause an open voltage to decrease, and it is impossible to form a high-quality amorphous silicon thin film, thus lowering the efficiency of the solar cell.
In particular, the case where the crystalline silicon substrate is subjected to texturing is unsuitable for the fabrication of ultra-thin HIT solar cells because a thick substrate is required due to the etching of the crystalline silicon substrate in the surface texturing process.
Accordingly, many attempts have been made to reduce the degree of surface etching and surface defects using an improved technique for etching a crystalline silicon substrate, but limitations are imposed on increasing the performance of the HIT solar cell.